When providing emergency cardiac patient care, it is essential to generate the patient's electrocardiograph (ECG) quickly and accurately for proper diagnosis and successful treatment. A typical ECG signal measuring system 10 is shown in FIG. 1. In this example, ECG signal measuring system 10 is part of diagnostic-quality monitor/defibrillator 12. To measure the ECG signal of a patient 14, ECG signal measuring system 10 is coupled to patient 14 through electrodes 15 and 16.
Large amplitude, low-frequency, non-physiological signals, commonly referred to as baseline wander, can saturate an ECG measurement system, resulting in the loss of patient ECG signal information. There are several sources of baseline wander including; DC bias currents, patient movement and changing patient impedance, which are described further below.
ECG signal measuring systems used for emergency medical applications typically use a DC bias current to detect disconnected electrode leads. This current interacts with the patient's impedance to cause a relatively high amplitude but low frequency signal that is superimposed on the relatively low voltage ECG signal when electrodes 15 and 16 are initially applied to patient 14. For convenience, this signal is referred to herein as the bias current signal. This bias current signal is illustrated in FIG. 2 by a curve 20. As can be seen in FIG. 2, an initial portion 21 of curve 20 has a relatively large rate of change. The bias current signal eventually begins to stabilize, as indicated by a portion 23 of curve 20. The bias current signal results in a significant rate of change of the combined input signal (i.e., the baseline wander combined with the patient ECG signal) during the initial period. This rate of change of the combined input signal is referred to herein as the slew rate. When the bias current signal eventually starts to stabilize, the slew rate of the combined input signal is reduced.
A similar problem is caused by movement of patient 14 or electrodes 15 or 16 that disturbs the electrical connection of electrodes 15 and 16 to patient 14. This movement can result in a significant change in the impedance presented to ECG signal measuring system 10. This change in impedance can result in a change in the bias current signal, which results in a slew rate of the combined input signal.
Baseline wander can also be caused by interaction of the bias current with changing patient impedance caused by the electrodes forming a better electrical connection to the patient over time.
Conventional techniques can be used to compensate for the "offset" caused by the baseline wander in order to keep the combined input signal from saturating the system. However, the inventors of the present invention have observed conventional compensation techniques are inadequate for the high slew rate of the combined signal caused during the initial period of the bias current signal.
FIG. 3 is a block diagram illustrative of conventional digital ECG signal measuring system 10 (FIG. 1). ECG signal measuring system 10 includes a preamplifier 31, a high pass filter (BPF) 33, an analog-to-digital converter (ADC) 35 and a second HPF 37. As will be appreciated by those skilled in the art, ECG signal measuring system 10 includes an anti-aliasing filter (not shown) configured to filter out frequency components of the input ECG signal above one-half of the sample rate of ADC 35.
In this example, the passband of HPF 33 is set at about 0.03 Hz, while the passband of HPF 37 is set at about 0.02 Hz. This gives a passband with a lower edge of 0.05 Hz. This performance is consistent with industry standards for diagnostic quality ECG systems (AAMI EC-11). Unfortunately, the baseline wander signal has frequency components above 0.05 Hz. Thus, in this example, HPF 33 passes the baseline wander signal along with the ECG input signal to cause the saturation problem described above.
One conventional solution to this problem is to increase the dynamic range of the system. Current industry standards require a dynamic range of at least 10 mV (i.e. ranging from .+-.5 mV). Diagnostic and interpretive algorithms require resolution of 5.0 .mu.V. This range is adequate for patients ECG signals that do not include baseline wander. Sources of baseline wander discussed above dictate that the dynamic range would have to be increased to greater than 150 mV. However, to increase the dynamic range and maintain a given resolution would require an increase in the number of bits of the analog-to-digital conversion. For example, a twelve-bit ADC can be used for 20 mV dynamic range and 5 .mu.V resolution. However, a sixteen-bit ADC may be required for 160 mV dynamic range and the same 5 .mu.V resolution. The cost of a sixteen-bit ADC is significantly higher than a twelve-bit ADC, which undesirably increases the cost of the ECG signal measuring system. Another solution to this problem is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/013,570, entitled "Digital Sliding Pole Fast Restore For An Electrocardiograph Display," Stice, et al. Although the digital sliding pole invention represents a substantial improvement over the prior art, further improvement is, of course, generally desirable. Thus, there is a need for a low cost ECG measuring system having a relatively large dynamic range and high resolution.